The invention relates to an electrically controlled pressure-regulating system for a hydraulic circuit.
It is designed especially for the brake systems of motor vehicles with anti-lock wheel braking. Present anti-lock systems employ, between a generator of fluid under pressure and an hydraulic motor, a solenoid valve controlled by a computer as a function of signals representing the rotation of the wheels of the vehicle, in order, in general terms, to relieve the pressure of the fluid in the hydraulic motor when the computer detects the imminent locking of a wheel and then to connect a second source of fluid under pressure so as to increase the pressure once again, until imminent locking is detected once more, the cycle then being repeated.
The solenoid valve used is a solenoid valve functioning in the all-or-nothing mode. Now the anti-lock periods when cycles, such as the abovementioned cycle, take place last a certain amount of time, during which the solenoid valve changes states very many times within a very short time. This gives rise to an unpleasant hammering noise caused by the moveable part of the solenoid valve.
Moreover, since the system functions in the all-or-nothing mode, pressure surges occur in the brakes, and these generate poorly controlled transient states.
The object of the present invention is to overcome these disadvantages by using a so-called proportional solenoid valve ensuring, in the working circuit, a hydraulic pressure which is a function of the current circulating in the coil of the solenoid valve, within a specific range of movement of the magnetic core of the solenoid valve.
The advantage of such a solenoid valve is, therefore, that it can be controlled simply by varying the current circulating in the coil, without the need for hammering; this thus results in a pronounced reduction of the pressure surges in the brakes. Moreover, such a solenoid valve can easily be controlled by means of a computer ensuring a variable-frequency cutoff of a direct current, such as is found on board a motor vehicle, or a fixed-frequency cutoff with a variable cyclic ratio, the intensity of the current integrated by the coil then being a function of the cyclic ratio.
Such a solenoid valve is described, for example, in U.S. Pat. No. 4,744,389. However, the solenoid valve described in this document cannot be arranged in a hydraulic anti-lock wheel-braking circuit because of the pressures adopted, especially during the energization of the solenoid valve. In fact, the solenoid valve of this document is designed to function permanently in a system for regulating a relatively low pressure. Indeed, the force generated by the return spring used directly opposes the hydraulic reaction force exerted on the slide. This physical limitation therefore precludes the use of such a solenoid valve for high pressures in the hydraulic motor.